1. Field of the Invention
This invention relates to a steering controlling device for a wheel crane which can change over the steering mode between a normal steering mode (hereinafter referred to briefly as normal mode) wherein only the front wheels are steered and another special steering mode (hereinafter referred to briefly as special mode) wherein either both of the front wheels and the rear wheels or only the rear wheels are steered.
2. Description of the Related Art
A wheel crane operates, at a site of operation, in a special mode wherein either both of the front wheels and the rear wheels or only the rear wheels can be steered in order for the wheel crane to make a small turn to assure a good running condition at a narrow location, but upon running on a public way, it is changed over to a normal mode wherein only the front wheels can be steered for safety.
It is to be noted that the special mode may be a so-called crab mode wherein the front and rear wheels are steered in the same direction or a so-called clamp mode wherein the front wheels and the rear wheels are steered in the opposite directions or else a rear mode wherein only the rear wheels are steered.
Conventional steering controlling devices for effecting such changing over control of the steering mode are generally divided into two types. A steering controlling device of the first type is constituted such that a locking pin is operated in response to an operation of a mode selecting switch provided for selection of a steering mode. More particularly, when a normal mode is selected by the mode selecting switch, the rear wheels are automatically locked, but when a special mode is selected, the rear wheels are unlocked automatically. Such steering controlling device is disclosed, for example, in Japanese Utility Model Publication No. 62-5975.
A steering controlling device of the second type is constituted such that changing over to a special mode is permitted on condition that a locking switch for changing over between a locking position and an unlocking position is operated to the unlocking position.
However, with a steering controlling device of the construction of the first type described above, since the rear wheels can be locked only in the normal mode wherein the rear wheels cannot be steered when the locking pin is out of position with respect to a locking hole, it is impossible to move the rear wheels to make positioning between the locking pin and the locking hole. Accordingly, a procedure must be followed that the steering mode is first returned to a special mode, and then the rear wheels are moved suitably to roughly adjust the locked positions thereof, whereafter the steering mode is returned to the normal mode to try a locking operation again. Therefore, a locking operation is very cumbersome.
It is to be noted that, in order to cope with this, a so-called centering correcting mechanism is additionally provided which directs, upon locking, the rear wheels to the direction of running movement of the wheel crane, and a locking operation is effected on condition that centering has been completed, that is, the locking pin and the locking hole are adjusted in position relative to each other. However, a limit switch for detecting completion of such centering has some tolerance in detection, and an accurately centered condition may not always be attained. As a result accurate positioning is difficult.
On the other hand, also in the case of a steering controlling device of the construction of the second type, since the normal mode is automatically set when the locking switch is operated to the locking position to effect locking, it is impossible to move the rear wheels to effect positioning for locking similarly as in the steering controlling device of the first type.
Besides, the steering controlling devices of both of the first and second types are constituted such that locking and change-over control of the steering mode can be carried out whether the crane is in a normal running condition or in a stopping or low speed running condition. Accordingly, there is the possibility that locking or changing over of the steering mode may take place in response to an operation error during normal running of the crane so that a dangerous condition may take place.